Conventional magnetic return paths for accelerometers, such as the accelerometer shown in FIG. 1, create a flux distribution in an air gap between an excitation ring and a pole piece that interacts with a coil that is attached to a flexible proof mass. The flux interacts with the current in the coil to produce a rebalance force proportional to the acceleration to which the device is subjected. The flux density across the air gap is not uniform given geometric constraints of constructing useful circuits. Further, the field strength of a magnetic circuit is not constant when it interacts with the coil with changing direction of current flow. The field strength follows the minor loop slope of the magnet. If the device is subjected to vibration which can change the orientation of the coil with respect to the flux and the amplitude of the flux itself, the output of the device will change independently of the acceleration being measured. This error is called vibration rectification error (VRE).
For any given magnetic circuit, there is an optimum location of the coil in the field to minimize VRE. Means have been developed to cope with this problem using spacers located between the coil and the proof mass. However, the spacers increase the pendulosity, add cost and increase the difficulty of manufacturing. Also, the desire to minimize the output change under vibration has led to the development of short coils that need to be extremely clean and uniformly manufactured to avoid contact with the components that define the air gap.
Typical manufacturing techniques present difficulties in producing a highly finished yet clean excitation ring because excitation rings are generally produced as one piece units that are difficult to machine and clean on their inner surface. This can result in small particles remaining that can interfere with proper operation of the accelerometer. Additionally, previous manufacturing techniques including those that use two piece excitation rings have a number of sources of possible error in the geometric configuration of the pole piece and magnet in relation to the coil, upper surface of the excitation ring, and proof mass. Bonding layers between the pole piece and magnet and between the magnet and excitation ring as well as the height of the pole piece and the height of the magnet are variable and can result in variability in the position of the pole piece and magnet between different accelerometers that results in increased vibration rectification error because the pole piece is not in an optimal position in relation to the other parts of the accelerometer.